Making a Crankshaft -3

by John

Yesterday I showed my model engineering group GSMEE, the bare old crankshaft, and the milled and marked lump of steel from which I am gradually removing all steel which is not crankshaft, and turning it into hot, sharp tiger snake repellant, I mean swarf.

“why don’t you just fix the old one?” (I tried. Unsuccessfully)

“why don’t you TIG weld the joins in the old one?” (possible, but Nah! It would distort, and would require finish turning, and I would probably be unable to use the existing gears, eccentrics, and big ends).

“You are going to get a lot more swarf”. (Yep!)

“What are the crosses for?” (So I know which bits to remove)

And some helpful suggestions….

“Cramp the steel vertically on the CNC mill, and drill the centers using CNC movements” (yep!)

“Machine the journals a bit oversize, and re-machine the bronze bearings because they will be worn, and a bit oval” (yep!)

“Turn between centres. Use a solid tailstock centre, not a bearing type” (Yep!)

“Turn the journals toward the headstock, and reverse the workpiece to complete the other side” (yep!)

And, considerable skepticism that the job would be completed successfully. (Understandable. It IS a challenge. But so far so good).

Today another half day workshop session. About 5 hours.

First task was to center drill the turning centers for the mainshaft and the big end journals. I marked the positions on a surface plate, using a height gauge, but the actual drilling using a center drill bit was determined by cramping the workpiece to a large angle plate, establishing a master face, and using CNC to locate the drilling positions. I was pleased to note that the CNC positions lined up pretty precisely with my measured positions. Then turned over the workpiece, cramping the same face to the angle plate, re-established zero X and Y positions , and drilled the other end. Those center drilled holes will determine the axes for the mainshaft and big ends, which are the essential reference points.
Deeply drilled holes for some heavy, interrupted turning sessions.

But before the turning there is a substantial amount of steel to be removed by other means.

“Other means”.

First I tried milling, using sharp carbide end mills 12mm diameter. After I had broken 2 newish end mills, I thought about other means.

First, I used a bandsaw to make deep cuts. That steel is 38.1mm thick, and the cut is 38mm long. That is the thickest steel that I have ever cut with a bandsaw. I was REALLY surprised how quickly the cuts were made. Each one took 70-90 seconds.

Then I thought about using the bandsaw to make the long cuts, up to 200mm long, through 38.1mm steel. But the bandsaw blade was 25.4mm wide, so I chose to make a milling slot 25.4mm wide to allow access of the bandsaw blade. In the process I broke a HSS then a solid carbide 12.7mm cutter. Expensive.

A bandsaw cut 200mm long, in 38.1mm thick steel. It took 7.5 minutes. Surprisingly quick and effective! But cutting the slot was problematic… broke two cutters.
So, for next one, I just used the bandsaw, making two angled cuts to remove 90% of the waste, then milling the hump out. That was easy. And quick! Way to go!.
After the tidy up milling. That is half of the debulking process. Another half to be done tomorrow. Already the lump of steel is a lot lighter. Currently 10.5kg. The old crankshaft weighs 3.3kg, so still a lot of material to be removed. Just a reminder that the original weight was 26.5kg!

SO. I have formed an opinion about removing waste metal. BANDSAWING BEATS MILLING, HANDS DOWN! (but for finishing, milling wins.)